Method for producing metallic iron particles

ABSTRACT

A method of producing iron particles in a generally vertical furnace in which a gravitational flow of particulate material is reduced in a reducing zone by a countercurrent flow of a reducing gas containing a reductant, the spent reducing gas is removed and cooled and a portion is introduced as cooling gas in a cooling zone near the bottom of the furnace, and a portion of the cooling gas is upgraded in reducing potential and introduced to the reducing zone as reducing gas. Apparatus is also provided for carrying out the method.

REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of my application Ser. No.611,099, filed Sept. 8, 1975, which is a continuation-in-partapplication of my application Ser. No. 578,477, filed May 19, 1975.

BACKGROUND OF THE INVENTION

The recent high cost of scrap as a feed material for steelmakingfurnaces has caused steelmakers to turn elsewhere for their rawmaterials. One recently attractive raw material is reduced iron in theform of sponge iron, iron particles, pellets, briquets, and the like,which has been produced by the direct reduction of iron oxides or ironores. Such materials will hereinafter be referred to collectively asmetallized pellets. These metallized pellets are well suited as feedmaterial, particularly to an electric arc steelmaking furnace. As aresult, there have been a number of processes developed for theirproduction. To be an attractive feed material, the pellets should be atleast 85 percent reduced, and preferably over 90 percent reduced.

U.S. Pat. No. 3,375,099 discloses a direct reduction process in whichiron ores are reduced in a shaft furnace by contact with hot reducinggases generated by the incomplete combustion of a mobile fuel, such asnatural gas, with oxygen. The spent reducing gases, which are also knownas top gases or off gases, are withdrawn from the shaft furnace, cooledand reintroduced at the bottom of the furnace as cooling gases to coolthe product. The cooling gas is then allowed to flow upwardly throughthe shaft furnace, thus creating a closed circuit. It is also known thatcooling of spent top gas enhances its reducing capacity. U.S. Pat. No.3,748,120 teaches an improved method for reducing iron oxide tometallized iron, in which a reducing gas is catalytically reformed froma mixture of gaseous hydrocarbon and spent reducing gas from thereduction process. Cooling gas is circulated through the bottom portionor cooling zone of a shaft furnace in a closed loop, that is, thecooling gas is not allowed to flow upwardly into the reducing zone. U.S.Pat. No. 3,799,521 teaches that allowing cooling gas to flow upwardlyinto the reducing zone of a shaft furnace is detrimental in that it doesnot permit fully independent control of the reduction and cooling stepsof the process. It further points out that to achieve a particulardesired degree of carburization, the composition and flow-rate of thecoolant gas should be controllable independently of the conditionsexisting in the reduction zone of the furnace.

When spent top gas is used for cooling the pellet product as shown inU.S. Pat. No. 3,375,099, the gas which flows upwardly into the reducingzone (upflow gas) is not fully preheated, because the flow of coolinggas in actual practice must exceed the theoretical quantity required.That is, the thermal capacity of upflowing gas must exceed the thermalcapacity of the descending burden. This precludes the cooling gas beingfully preheated by the hot burden. Hot, fresh reducing gas enters thereducing zone through bustle pipes around the perimeter of the furnaceforcing the upwardly flowing cooling gas to the center of the furnace,which results in cooling the central portion of the burden in thereducing zone. Further, when the spent top gas is not upgraded prior tointroduction as cooling gas, it has poor reducing potential when itenters the reduction zone. These two factors combine to cause incompletereduction of the burden with a resulting lesser metallization of theproduct.

OBJECTS OF THE INVENTION

It is the principal object of my invention to provide an improved methodfor directly reducing particulate metal oxide material to a metallizedproduct in a shaft furnace in which the spent top gas is utilized ascooling gas, with a portion of the cooling gas being further utilized asa reductant in the reducing zone of the shaft furnace.

It is another object of my invention to provide a method of reformingupflow gas from the cooling zone to reducing gas.

It is another object of my invention to provide a method of upgrading atleast a portion of the cooling gas in reducing potential prior toutilizing it as reducing gas.

It is also an object of my invention to provide a method of enrichingthe spent top gas with a gaseous hydrocarbon prior to utilizing theenriched gas as a cooling gas.

It is another object of my invention to provide apparatus for carryingout the methods.

It is a further object of my invention to provide a simplified apparatusfor carrying out the methods with only a single treatment apparatus forboth top gas and cooling gas.

BRIEF DESCRIPTION OF THE DRAWINGS

My invention is better understood by referring to the following detailedspecification and the appended drawings in which:

FIG. 1 is a schematic drawing of a vertical shaft furnace and itsassociated equipment for utilizing one method of upgrading the reducingpotential of a portion of cooling gas prior to utilizing it as areducing gas.

FIG. 2 is a schematic drawing similar to FIG. 1 showing two alternativemethods of upgrading the reducing potential of a portion of the coolinggas prior to utilizing it as a reducing gas.

FIG. 3 is a schematic drawing of a vertical shaft furnace similar toFIGS. 1 and 2, but employing a simplified gas cooling and recyclingsystem wherein the spent top gas is reformed to reducing gas andreturned to the reducing zone of the furnace.

DETAILED DESCRIPTION

A direct reduction process has been developed for producing high qualitymetallized pellets with an extremely high degree of thermal efficiency.

The process employs a vertical shaft type furnace having a reducing zonein the upper region of the furnace and a cooling zone in the lowerregion of the furnace. Hot reducing gas from any external source isintroduced to the reducing zone. For the purpose of overall processdescription, the reducing gas utilized herein consists principally of COand H₂ produced by the continuous catalytic reforming of a hydrocarbonsuch as natural gas, petroleum distillates, methane, ethane, propane,butane, or other readily vaporizable hydrocarbon. The continuouscatalytic reforming is accomplished in a reforming furnace which employsan indirectly heated catalyst bed. The metallized pellets are cooled byrecirculating a cooling gas through a cooling gas circuit in the coolingzone of the reduction furnace. Top gas from the reduction furnace isadmitted to this circuit. The process embraces improvements in thetechnology of cooling directly-reduced metallized pellets, which isimportant not only with respect to thermal efficiency of the process,but also with respect to obtaining a high degree of metallization of thepellets in a reasonable time. The process is extremely well suited forthe production of iron-and-steel-making grade metallized pellets. Itwill therefore be so described.

The metallized product, which is at least 85 percent reduced andpreferably at least 90 percent reduced is produced in a generallyvertical shaft furnace having an upper reducing zone and a lower coolingzone. A gravitational flow of metal oxide material or burden isestablished by charging particulate metal oxide material to the upperportion of the furnace and removing the metallized product from thebottom of the furnace. A hot reducing gas having CO and H₂ as reductantcomponents is introduced to the flow of material through a bustle pipeand tuyere inlet system intermediate the ends of the furnace, flowscountercurrent through the material, reducing a substantial portion ofthe metal oxide, and forms a top gas. The top gas is removed from theupper portion of the furnace, cooled, and divided into two portions. Thefirst portion is introduced to a cooling gas circuit which introducescooling gas to the cooling zone through an inlet near the lower end ofthe furnace. The cooling gas flows upwardly and a portion of it isremoved at the top of the cooling zone, scrubbed and cooled, andrecirculated in a closed loop. Cooled top gas (make-up gas) is added tothe removed cooling gas and the mixture is directed to the furnacethrough the cooling gas inlet. An amount of cooling gas substantiallyequal to the amount of make-up gas flows upwardly into the reducingzone, is heated by the hot particulate material, and acts as reducinggas. To effectively cool the hot burden, the flow of cooling gasadmitted to the cooling zone must exceed the theoretical quantity, i.e.,the thermal capacity of the cooling gas must exceed the thermal capacityof the descending burden. To effectively preheat the portion of coolinggas which flows upwardly into the reducing zone, the flow of thisportion must be less than the theoretical quantity, i.e., the thermalcapacity of the descending burden must exceed the thermal capacity ofthe upflow gas. Thermal capacity of a substance is the product of thespecific heat of the substance times the flow. The specific heat unitscould be expressed as BTUs per pound of substance, and the flow units aspounds per hour. Thus, the thermal relationship of gas to burden can beexpressed:

    C.sub.g × W.sub.g × ΔT.sub.g  = C.sub.b × W.sub.b × ΔT.sub.b

Where:

C_(g) = gas specific heat

W_(g) = gas flow

ΔT_(g) = gas heat change in degrees

C_(b) = specific heat of the burden

W_(b) = flow of the burden

ΔT_(b) = burden heat change in degrees

Specific heat is a constant for each substance.

To effectively cool the burden in the cooling zone, the gas flow raterelative to the burden flow rate must be adjusted so the gas heat changein degrees will be less than the burden heat change in degrees, as willbe readily understood by one skilled in the art of gas to solidscounterflow heat exchange. Similarly, to effectively and fully preheatthe portion of cooling gas which upflows into the reducing zone, thisupflow gas flow rate relative to the burden flow rate must be such thatthe gas heat change in degrees is greater than the burden heat change indegrees. The cooling arrangement of the present invention comprehendsboth effective cooling of the burden and effective preheating of theportion of cooling gas flowing upwardly into the reducing zone. Thesecond portion of the cooled top gas may be introduced to a reformerfurnace as a fuel to heat a catalyst in a tube in such furnace. Agaseous hydrocarbon and steam are passed through the catalyst, forming areductant-containing reducing gas which is introduced to the reductionzone of the furnace through the tuyere inlet.

Referring now to FIG. 1, a vertical shaft furnace 10 has a feed hopper12 mounted at the top thereof into which iron oxide pellets 14 or othermaterial such as lump ore are charged. The pellets descend by gravitythrough one or more feed pipes 16 to form a bed 18 of particulate ironoxide containing material, or burden, in the shaft furnace. The upperportion of the shaft furnace 10 comprises a reducing zone while thelower portion of the furnace comprises a cooling zone. A pelletdischarge pipe 20 is located at the bottom of the shaft furnace 10.Reduced material is removed from the furnace by discharge conveyor 22located beneath discharge pipe 20. Removal of the metallized pelletsfrom discharge pipe 20 establishes gravitational flow of the particulateiron oxide burden in shaft furnace 10.

At the upper portion of the shaft furnace 10 is a bustle and tuyeresystem, indicated generally at 24, having gas ports 28 through which hotreducing gas is introduced to flow upwardly in counterflow relationshipto the movement of the burden 18. The spent top gas exits the furnacethrough gas takeoff pipe 30 at the top of the furnace. The lower end ofpellet feed pipe 16 extends below takeoff pipe 30, which arrangementcreates a reacted gas disengaging plenum 32 which permits the gas toexit generally symetrically from the pellet stock line 34 and flowfreely to the takeoff pipe 30.

A cooling gas loop recirculating circuit is provided at the cooling zoneof the furnace to final cool the pellets prior to their discharge. Thiscircuit includes a scrubber-cooler 36, a recirculating gas blower 38,flow-control valve 39, a gas inlet 40 and a gas outlet 42. The blower 38is located in inlet pipe 44 leading from the scrubber-cooler to theinlet 40. Inlet 40 leads to a gas distributing member 46 located withinthe furnace 10. Cooling gas collecting member 48 is positioned above thegas distributing member 46 and is connected to scrubber-cooler 36 bypipe 50. Generally, that portion of the furnace, between and includingmembers 46 and 48, comprises the cooling zone which forms an integralpart of the cooling gas loop recirculating circuit.

A reformer furnace 54, having fuel fired burners 56, a flue pipe 58 anda plurality of indirect heat exchanger catalyst tubes 60, which areexternally heated, only one being shown, generates hot reducing gas. Thereducing gas flows from the catalyst tubes 60 to the bustle and tuyeresystem 24 through gas pipe 62.

The spent top gas, leaving the shaft furnace 10 through the takeoff pipe30, flows to a scrubber-cooler 64 wherein the gas is cooled and the dustparticles are removed. Pipe 66 leads from scrubber-cooler 64 to a gasblower 68. Pipe 66 contains a valve 70 in the line for venting cooledtop gas via vent Y, if such is desired. Blower 68 is required tocirculate the top gas from the scrubber-cooler through pipes 72, 74, and76. Pipe 72 admits a portion of the top gas to the cooling gasrecirculating system at cooling gas pipe 44. Pipe 74 connects to gaspipe 62 to introduce scrubbed and cooled top gas to the hot reformed gasfrom the reformer furnace to reduce its temperature prior to introducingit to the reduction furnace. Pipe 76 transmits the remaining spent topgas to the reformer furnace as fuel to be used as a source of heat.

A source of a gaseous hydrocarbon such as natural gas delivers such gasto burner 56 through pipe 78 having a flow control valve 80 therein.Combustion air from the burner 56 in the reforming furnace is suppliedfrom source A through pipe 82 having a flow control valve 84 therein.Steam from source S and a gaseous hydrocarbon from source N areintroduced to the catalyst tubes 60 through pipes 88 and 90 respectivelyeach of which has a flow control valve therein. Alternatively, thereforming oxidant from source S can be CO₂ and water vapor from spentreducing furnace top gas.

A temperature sensing element 94 adjacent reducing gas inlet 26 controlsthe flow of top gas through flow control valve 96 in pipe 74 to balancethe flows of hot reducing gas from the reformer and the cooled top gasso the reducing gas mixture entering inlet 26 will be at the desiredtemperature. Temperature sensing element 98 located adjacent cooling gasoutlet 42 controls valve 39 to maintain the desired exit temperature ofthe cooling gas at outlet 42.

The hot reducing gas admitted to the shaft furnace 10 through gasintroduction ports 28 has a reductant (H₂ + CO) to oxidant (H₂ O + CO₂)ratio of about eight. The spent top gas in pipe 66 after having beenscrubbed and cooled in scrubber-cooler 64 has a reductant to oxidantratio of about five, containing for example 14% CO₂, 3% H₂ O, and 83%H₂ + CO. Because it has a lower reductant to oxidant ratio, the spenttop gas, even after cooling, is a poor quality reducing gas having poorreducing potential.

It has been found that reduced iron pellets are a good catalyst for thewell-known reversible water-gas shift reaction.

    CO + H.sub.2 O ⃡ CO.sub.2 + H.sub.2

at a temperature of about 800° to 1100° F., and preferably about 1000°F., the water-gas shift reaction appreciably lowers the CO₂ content ofthe cooling gas in the cooling zone while simultaneously raising the H₂O content an equivalent amount. The water vapor thus produced iscondensed and removed in the scrubber-cooler 36. The gas which re-entersthe cooling zone through distributor 46 and flows upwardly from thecooling zone into the reducing zone as indicated by arrows 102 has areductant to oxidant ratio of about 6.5. Although the preferred coolinggas temperature as monitored at temperature sensing element 98 is about1000° F., the process operates well at temperatures from about 800° toabout 1100° F. (about 425° to about 600° C.).

Table 1 shows the effect of temperature of the cooling gas at outlet 42on the reductant to oxidant ratio of the cooling gas allowed to flowupwardly as reductant. In this example, the reductant to oxidant ratioof the spent top gas entering the cooling system through pipe 72 is5.25. Note that the ratio of thermal capacities of gas to burden isconstant above the cooling zone, but varies in the cooling zone. Thisoccurs because the specific heat of each substance, burden, and gas,varies with temperature. The temperature of the burden as it enters thecooling zone is always about the same, the exit temperature of thecooling gas varying according to the cooling gas flow rate.

Whereas in prior processes, cooling gas allowed to flow upwardly throughthe burden as reductant was not fully heated when reaching the reducingzone and cooled the burden center, the invented process overcomes thatdisadvantage. By controlling cooled top gas added to the cooling looprecirculating circuit through flow control valve 35, the upwardlyflowing gas 102 is fully preheated by the descending hot particulatematerial before the gas enters the reduction zone.

Thus, the upflow gas is upgraded in three stages: first, the spent topgas is cooled to remove vapor and increase its reducing capacity;second, water vapor formed in the cooling zone is removed from theremoved cooling gas, whereby the reductant to oxidant ratio is furtherincreased; and third, the upflow gas is preheated by the descendingburden to the required preheat temperature before it enters the reducingzone.

In an alternative embodiment shown in FIG. 2, the portion of top gaswhich is admitted to the cooling circuit through pipe 72 does not passthrough flow control valve 35, but instead is passed through flowcontrol valve 106 and a CO₂ removal tower 108 which can be a part of aconventional CO₂ removal system such as the commonly usedmonoethanolamine system. By removing CO₂ from this portion of top gasprior to admitting it to the cooling zone circuit, its reducingpotential is increased external of the cooling zone. The upflow gaswhich enters the reducing zone thus has good reducing potential and theflow rate of upflow gas relative to the descending burden flow rate ismaintained in proper relationship to insure adequate preheating of theupflow gas.

In the second alternative embodiment of FIG. 2, valve 106 is closed andvalve 35 is opened to admit top gas to pipe 110. Natural gas, or otherhydrocarbon vapor from a source 112, is admitted to pipe 110 throughpipe 114. The flow of this hydrocarbon vapor is regulated by valve 116.The portion of top gas which is admitted to the cooling zone circuitthrough pipes 72 and 110 contain CO₂ and residual water vapor, both ofwhich are reforming oxidants for the reforming of a hydrocarbon such asmethane to form CO and H₂. The well-known methane reforming reactionsare as follows:

    CH.sub.4 + CO.sub.2 → 2H.sub.2 + 2CO

    ch.sub.4 + h.sub.2 o → 3h.sub.2 + co

u.s. pat. No. 3,375,098 discloses the addition of a hydrocarbon vapor toa portion of cooled top gas from a shaft type reduction furnace. Thismixture of hydrocarbon vapor and cooled top gas is then admitted to alower region of the shaft furnace to serve as the cooling gas, and is inturn permitted to upflow into the reduction zone where some reformingwill occur, thus upgrading the upflow gas in reducing potential.However, with the arrangement disclosed in U.S. Pat. No. 3,375,098, theamount of reforming is insufficient to be very effective since the largeamount of cooling gas required to adequately cool the descending burdenis too great to be properly preheated to accomplish any meaningfulamount of upflow gas reforming.

In the present invention, the effective final cooling of the burden isaccomplished independently of the amount of upflow gas by means of thecooling loop recirculating circuit as described above. The flow rate ofcooled, hydrocarbon enriched, top gas admitted to the cooling circuit iscontrolled, relative to the burden flow rate, to insure the upflow gasbeing fully preheated and reformed to upgrade its reducing potential.

It should be noted that the introduction of cooled gas into the coolingloop recirculating circuit can be at any point in the loop in additionto those shown in the drawing. For instance, the cooled top gas may beintroduced directly to the cooling zone, or to the circuit, eitherbefore or after cooler-scrubber 36. Of course, the cooled top gas may beeither in the untreated, enriched, or CO₂ -removed forms.

In the embodiment depicted in FIG. 3, the cooling gas loop recirculatingcircuit is integrated with the spent top gas cleaning and recyclingcircuit to drastically reduce the amount of piping and number of pumpsrequired in the system as well as to eliminate the separate cooling zonescrubber-cooler and compressor which are present in the embodiments ofFIGS. 1 and 2. Spent top gas exits furnace 10 through gas take-off pipe130, flows to scrubber-cooler 132 wherein the gas is cooled and dustparticles are removed. A first portion of the gas exitingscrubber-cooler 132 is directed to blower 134 through pipe 136. Thisportion of the spent top gas is further divided, a portion which acts ascooling gas passing through pipe 138 and a second portion sometimesknown as process gas, recycle gas or reforming oxidant passing throughpipe 140 to reformer 54.

Cooling gas outlet pipe 145 connects cooling gas outlet 42 toscrubber-cooler 132. This may be done by connection with spent top gastake-off pipe 130.

The second portion of gas exiting scrubber-cooler 132 passes throughpipe 147 to burner 56 of reformer furnace 54 where it is burned as fuelto heat the reformer. Natural gas from source 152 can be added to thecooling gas in pipe 138 through pipe 154 which has flow control valve156 therein.

The embodiment of FIG. 3 is equally applicable to the direct reductionprocess without upflow reforming as it is to the invented processdescribed herein which is characterized by upflow reforming.

As can readily be seen from the foregoing, I have invented an improvedprocess for the direct reduction of metal oxides to metallized particleswith a greater thermal efficiency than heretofore possible.

                                      TABLE 1                                     __________________________________________________________________________             Cooling Gas Flow                                                                        Ratio of Thermal                                                    Rate Through the                                                                        Capacity of Gas     Ratio of Thermal                                Cooling Zone in                                                                         in Cooling Zone                                                                          Gas Upflow in                                                                          Capacity of Upflow                              Normal Cubic                                                                            to Thermal Normal Cubic                                                                           Gas to Thermal                                                                           Reductant to                Exit Temperature                                                                       Meters Per                                                                              Capacity of                                                                              Meters Per                                                                             Capacity of                                                                              Oxidant Ratio               of Cooling Gas                                                                         Metric Ton                                                                              Descending Burden                                                                        Metric Ton                                                                             Descending Burden                                                                        of Upflow                   __________________________________________________________________________                                                      Gas                         800 F (425 C)                                                                          922       1.83       318      0.5        5.96                        900 F    804       1.60       328      0.5        6.32                        950 F    755       1.51       334      0.5        6.43                        1000 F   711       1.42       341      0.5        6.48                        1050 F   672       1.35       351      0.5        6.47                        1100 F (600 C)                                                                         637       1.28       363      0.5        6.41                        __________________________________________________________________________

What is claimed is:
 1. A method for producing a metallized productcomprising:a. establishing a gravitational flow of particulate metaloxide material by charging the particulate metal oxide material to theupper portion of a generally vertical furnace having an upper reducingzone and a lower cooling zone, and removing the metallized product fromthe bottom of the furnace; b. introducing a reducing gas to thegravitational flow of material at a temperature sufficient to promote areducing reaction between said reducing gas and said material at a firstinlet intermediate the ends of the furnace; c. causing said reducing gasto move countercurrent through the gravitational flow of material, reactwith and reduce a substantial portion of the metal oxide and form a topgas; d. removing said top gas from the upper portion of the furnace; e.cooling said top gas; f. introducing a portion of said cooled top gas ascooling gas into a second inlet near the lower end of said furnace; g.removing a first portion of said cooling gas from said furnace at alocation intermediate said first and second inlets; h. adding saidremoved portion of said cooling gas to said top gas prior to step (e);and i. causing a second portion of said cooling gas to flow upwardlythrough the gravitational flow of material, become heated thereby, andact as a reducing gas in the reducing zone;whereby that portion of saidcooling gas flowing upwardly through the gravitational flow of materialhas a greater reducing potential than the spent top gas, and is thus aneffective reductant.
 2. A method according to claim 1 wherein saidreducing gas is a reformed vaporizable hydrocarbon.
 3. A methodaccording to claim 2 wherein said hydrocarbon is selected from the groupcomprising natural gas, petroleum distillates, methane, ethane, propane,butane.
 4. A method according to claim 1 wherein said removed and cooledtop gas is separated into a first portion and a second portion, and saidsecond portion is introduced as a fuel into a furnace containing a tubehaving catalyst therein.
 5. A method according to claim 1 wherein saidremoved and cooled top gas is separated into a first portion and asecond portion, and said second portion is introduced to a catalyticreformer as a reforming oxidant.
 6. A method according to claim 4wherein said second portion of said cooled top gas is mixed with agaseous hydrocarbon to form the fuel mixture to heat said catalyst.
 7. Amethod according to claim 5 wherein said second portion of said cooledtop gas is mixed with a gaseous hydrocarbon to form reforming oxidantmixture.
 8. A method according to claim 1 further comprising mixing asecond portion of said cooled top gas with said reducing gas andintroducing the mixture to said first inlet whereby the resultingmixture will have the proper proportion of hot reducing gas and cooledtop gas to bring the temperature of the mixture to the desired inlettemperature.
 9. A method according to claim 1 wherein said particulatemetal oxide material is iron oxide.
 10. A method according to claim 1further comprising controlling the rate of flow of cooling gasintroduced to said second inlet to maintain the temperature of theportion of said cooling gas being removed from said furnace at betweenabout 800° to 1100° F.
 11. A method according to claim 1 furthercomprising adding a gaseous hydrocarbon to said cooling gas prior tointroducing said cooling gas into said second inlet, whereby said addedgaseous hydrocarbon is reformed in the furnace to a highly effectivereductant.
 12. A method for producing metallized pellets comprising:a.establishing a gravitational flow of metal oxide material in a verticalshaft furnace; b. introducing a gaseous reductant at an intermediatelocation within said furnace through a first inlet whereby the gaseousreductant moves in counterflow relationship with the descending metaloxide burden; c. removing the top gas which is the reacted gaseousreductant from the upper portion of the furnace; d. cooling said topgas; e. introducing a portion of the cooled top gas as a cooling gasdirected into the vertical shaft furnace at a second inlet below saidfirst inlet whereby said cooling gas is in direct heat exchangerelationship with the descending burden; f. removing a portion of thecooling gas at an outlet intermediate to said first and second inlets;and g. adding said removed cooling gas to said removed top gas prior tostep (d);whereby the cooling gas, upon reacting with the metallizedburden between said second inlet and said outlet have a greater reducingpotential than does the spent top gas.
 13. A method according to claim12 further comprising adding a gaseous hydrocarbon to said cooling gasprior to introducing said cooling gas into said second inlet, wherebysaid added gaseous hydrocarbon is reformed in the furnace to a highlyeffective reductant.